1. Field of the Invention
This invention is directed to a high temperature carbonaceous cement for bonding large carbon bodies which is curable at a low temperature while being capable of retaining strength and stability at extremely high temperatures.
2. Description of Related Art
Large machined graphite shapes have many uses and may be subject to extreme temperature conditions and/or thermal cycling. For example, in the aerospace industry graphite is used in large molds for the production of composite structures. Such large shapes, due to their sheer size, are made from smaller parts and assembled together. The assembly of the graphite shapes requires carbonaceous bonding agents which must withstand extreme temperatures and thermal cycling while retaining the strength of the joint. The strength of the joints between the individual graphite shapes should be sufficient to avoid failure of the mold.
An example of a conventional carbonaceous cement is disclosed in U.S. Pat. No. 5,002,981 ('981 patent) to Chiu and is assigned to the assignee of the current invention. The cement of the '981 patent is made from a binder comprising a pre-imidized oligomeric precursor which polymerizes at the acetylenic end-caps of the molecule. Dynamic Mechanical Analysis (DMA) of the binder phase of the '981 patent after curing at 240.degree. C. for 2 hours shows a glass transition temperature of 274.degree. C. It also partially softens when heated above the 240.degree. C. cure temperature indicating that it had not completely gelled. Gelation of the binder occurs when it will no longer melt when heated above the initial cure temperature. Gelation indicates further cross-linking of the binder phase producing a stronger binder, thus a stronger cement. Thus, it is desirable to provide a carbonaceous cement for use at high service temperatures which is more completely gelled during its initial cure and substantially gels with less softening during subsequent heating.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a carbonaceous cement having superior strength and stability at high service temperatures.
It is another object of the present invention to provide a carbonaceous cement having a low curing temperature while retaining its strength at high service temperatures.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.